[release/8.0] JIT: DNER multiregs with SIMD12 (#91878)

* JIT: DNER multiregs with SIMD12s

Locals with SIMD12 fields will never match the ABI when they end up
as multireg returns, so these should always be DNER'd.

Fix #91214

* Fix test build

src/coreclr/jit/lower.cpp

@@ -7484,6 +7484,28 @@ bool Lowering::CheckMultiRegLclVar(GenTreeLclVar* lclNode, int registerCount)
             if (registerCount == varDsc->lvFieldCnt)
             {
                 canEnregisterAsMultiReg = true;
+
+#ifdef FEATURE_SIMD
+                // TYP_SIMD12 breaks the above invariant that "we won't have
+                // matching reg and field counts"; for example, consider
+                //
+                // * STORE_LCL_VAR<struct{Vector3, int}>(CALL)
+                // * RETURN(LCL_VAR<struct{Vector3, int}>)
+                //
+                // These return in two GPR registers, while the fields of the
+                // local are stored in SIMD and GPR register, so registerCount
+                // == varDsc->lvFieldCnt == 2. But the backend cannot handle
+                // this.
+
+                for (int i = 0; i < varDsc->lvFieldCnt; i++)
+                {
+                    if (comp->lvaGetDesc(varDsc->lvFieldLclStart + i)->TypeGet() == TYP_SIMD12)
+                    {
+                        canEnregisterAsMultiReg = false;
+                        break;
+                    }
+                }
+#endif
             }
         }
     }

src/tests/JIT/Regression/JitBlue/Runtime_91062/Runtime_91062.cs

@@ -1,5 +1,5 @@
 // Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.aa
+// The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Runtime.CompilerServices;
 using System.Runtime.Intrinsics;

src/tests/JIT/Regression/JitBlue/Runtime_91170/Runtime_91170.cs

@@ -1,5 +1,5 @@
 // Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.aa
+// The .NET Foundation licenses this file to you under the MIT license.
 
 // Found by Antigen
 

src/tests/JIT/Regression/JitBlue/Runtime_91214/Runtime_91214.cs

@@ -0,0 +1,40 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+// Found by Antigen
+
+using System.Runtime.CompilerServices;
+using System.Runtime.Intrinsics;
+using System.Numerics;
+using Xunit;
+
+public class Runtime_91214
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        Method0();
+    }
+
+    struct S
+    {
+        public Vector3 v3;
+        public bool b;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static S Method2()
+    {
+        return default;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void Method0()
+    {
+        S s = Method2();
+        Log(null, s.v3);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void Log(object a, object b) { }
+}

src/tests/JIT/Regression/JitBlue/Runtime_91214/Runtime_91214.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>